1. Field of the Invention
The invention relates to a method for managing power distribution in a hybrid electric vehicle and for obtaining an estimate of engine power for use in determining a desired vehicle traction wheel torque.
2. Background Discussion
A so-called power split hybrid electric vehicle powertrain has two power sources and a parallel power flow path under a first set of operating conditions and a series power flow path under another set of operating conditions. A first power source is an engine and generator subsystem, including a planetary gear set that mechanically couples the engine and the generator. A second power source is an electric drive comprising a motor/generator/battery subsystem. The battery is an energy storing medium for the generator and the motor. The generator and the motor can be referred to as electric machines since either is capable of acting as a motor under certain driving conditions and as a generator under other driving conditions.
A vehicle system controller will maintain the vehicle at its maximum performance value by managing the power distribution among the various components of the vehicle powertrain. It manages the operating states of the engine, the generator, the motor and the battery to maximize total vehicle efficiency.
The vehicle system controller will ensure that power management for optimum vehicle performance and drivability is maintained as it commands a desired engine torque and interfaces with a transmission control module. Based upon a commanded wheel torque and engine speed, the transmission control module determines a motor torque to effect a desired wheel torque. It also commands a generator torque to control the engine to achieve a desired engine speed.
For the purpose of disclosing the present invention, reference will be made to a so-called series-parallel hybrid powertrain configuration, although the invention is not limited to use in a powertrain configuration of that type.
In a series-parallel powertrain configuration, a motor torque control must coordinate a wheel torque command and a generator torque command. It should compensate for the inertia effect of the generator and the gearing elements to which the generator is connected. This will ensure that the vehicle system controller wheel torque command is fulfilled without wheel torque fluctuations. In the alternative, the control should use control algorithms that use variables not affected by inertial torque of the powertrain elements.
In a hybrid electric vehicle of this type, the battery power must be controlled under various driving conditions, including transient torque demands during accelerator “tip-in” and “tip-out” events, as well as steady-state driving. This must be done within the battery maximum power limits in order to avoid system failures and battery damage and to increase battery life.
The vehicle system controller in a hybrid electric vehicle powertrain will interact with the motor, the generator and the engine by requesting a desired wheel torque, a desired engine speed and a desired engine torque. One of the steps that can help in controlling the battery to prevent a command for battery power in excess of the maximum battery power limits is to correctly estimate the engine power.
The vehicle system controller should use a strategy that will take into account the functions of the generator, the engine, the motor, and the various system controllers as driving conditions change. This can be done by correctly calculating an estimate for engine power to ensure that the battery power stays within the maximum battery power limits under all conditions, including accelerator tip-in events, tip-out events and other transient torque demand events, as well as during steady-state driving.